Traditionally, MRI machines are large expensive devices which have to be located in a specially constructed or adapted MRI room and require the operator to have a high degree of skill. These aspects preclude the use of MRI as a diagnostic tool in applications where space is at a premium and where it would be desirable for the machine to be operated by, for example, nursing staff with only a limited degree of training. An example of such an application would be in an accident and emergency (A & E) unit.
To meet the aforementioned requirements, a magnet design should ideally have one or more of the following attributes:                Open, to facilitate patient handling and to minimise the risk of patient claustrophobia;        Compact and light, for ease of installation;        Small fringe field, to minimise the need for magnetic shielding of the room;        Low power consumption thus, low cryocooler power supply requirements;        Low cost; and        Suitable for stand-by use, allowing rapid field ramp-up and ramp-down.        
An impediment to such a design is the strong attractive force between the primary field coils and the magnet yoke which may severely limit the achievable performance and practicality of the magnet design.
The present invention addresses a solution to this problem by provision of a counterbalancing member which extends beyond the plane of a coil. It is known for pole pieces in MRI machines to be formed with a “lip” which extends over the outer surface of the coils, such as disclosed in GB-A-2 282 451 but conventionally, this lip is not fabricated or configured so as to counterbalance the aforementioned forces.